This little mill will have a ground floor of stone and an upper floor of log construction. My first thought was to make the entire structure from balsa foam. In doing some experimenting I found that while the lower section of stone will work fine, the upper log section did not look as good. I found that making the logs from balsa foam and gluing them to s foam core form was both easier to do and looked much more realistic.
This is the balsa foam base to be carved to represent stone.
This is the foam core upper section to be modeled as log construction.
Here are a couple of photos of the stone part with the stones carved into it. Carved is really not a good term but I'm not sure what to call it. Inscribed is better I guess. The softer balsa foam was used. It will get two coats of Gesso which will fill in the lines between stones a bit to look better in my view. It also had some unevenness added by pressing the surface with broken rock before inscribing the stone detail.